Marine riser tower

ABSTRACT

This invention relates to a marine riser tower ( 112, 114 ) for use in the production of hydrocarbons from offshore wells. The riser tower ( 112, 114 ) includes a plurality of fluid conduits, which may comprise production flow lines (P), gas-lift lines (G), water injection lines (W) and/or umbilicals (U). The conduits are supported in a single structure, and at least one of said conduits is provided with its own insulation within said structure.

The present invention relates to a marine riser tower, of the type usedin the transport of hydrocarbon fluids (gas and/or oil) from offshorewells. The riser tower typically includes a number of conduits for thetransport of fluids and different conduits within the riser tower areused to carry the hot production fluids and the injection fluids whichare usually colder.

The tower may form part of a so-called hybrid riser, having an upperand/or lower portions (“jumpers”) made of flexible conduit U.S. Pat. No.6,082,391 proposes a particular Hybrid Riser Tower consisting of anempty central core, supporting a bundle of riser pipes, some used foroil production some used for water and gas injection. This type of towerhas been developed and deployed for example in the Girassol field offAngola. Insulating material in the form of syntactic foam blockssurrounds the core and the pipes and separates the hot and cold fluidconduits. Further background is to be published in a paper Hybrid RiserTower: from Functional Specification to Cost per Unit Length by J-FSaint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, 18 Oct. 2001.

The foam fabrication and transportation process is such that the foamcomes in elements or blocks which are assembled together in theproduction at a yard. The fit of the elements in the tower is such thatthere will be gaps resulting from fabrication and assembly tolerances. Areadably flowable fluid, such as seawater, takes the place of air inthese gaps and a natural convection cycle develops. Natural convectionunder the form of thermosiphons can result in very high thermal losses.

When a riser tower houses both hot flowlines and cold water injectionlines, cold seawater surrounds the water injection lines up to the topof the tower. Upon shutdown this cold water naturally descends to bereplaced by warmer seawater surrounding the flowlines. This colder fluidaccumulates around the conduits such as the production line at thebottom of the tower, and accelerates the heat transfer from theproduction fluid in the conduit. This makes it difficult to meet thecooldown time criteria of the riser, locally.

Measures such as gaskets may be provided to break up this convection buthave only limited success, and add to the expense of the construction.

GB-A-2346188 (2H) presents an alternative to the hybrid riser towerbundle, in in particular a “concentric offset riser”. The riser in thiscase includes a single production flowline located within an outer pipe.Other lines such as gas lift chemical injection, test, or hydrauliccontrol lines are located in the annulus between the core and outerpipe. The main flow path of the system is provided by the central pipe,and the annular space may be filled with water or thermal insulationmaterial. Water injection lines, which are generally equal in diameterto the flowline, are not accommodated and presumably require their ownriser structure.

EP-A-0467635 discloses a thermal insulting material for use in pipelinebundles an pipeline riser caissons. The material is a gel-based materialthat may be used to fill the space between the lines in the riser.

The aim of the present invention is to provide a riser tower having areliable thermal efficiency and/or greater thermal efficiency for agiven overall cost. Particular embodiments of the invention aim inparticular to eliminate heat transfer by convection within and aroundthe tower, to achieve very low heat transfer. Particular embodiments ofthe invention aim for example to achieve heat transfer rates of lessthan 1 W/m²K.

The invention in a first aspect provides a riser tower wherein aplurality of rigid fluid conduits including at least one productionflowline are supported in a single structure, at least one of saidconduits being provided with its own insulation within the structure.

In particular embodiments, insulated lines are used for of productionflowlines and preferably also for gas lift lines. Insulation may beprovided also for injection lines, depending on actual temperatureoperating conditions.

A particular application of the present invention is in Hybrid RiserTowers, for example of free-standing type, where flexible lines areconnected to the riser at top and/or bottom.

The insulation may serve instead of or in addition to buoyant materialsurrounding the riser as a whole.

The insulation may take the form of a coating applied to the conduit, adual-wall (pipe-in-pipe) structure or a combination of both.

The riser tower may include a tubular Postural core. One or more of theconduits (such as production and/or gas lift lines) may be locatedinside the core, to isolate it further from the environment and thewater lines. This feature is the subject of a co-pending application.

These and ether advantageous features are defined in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, by reference to the accompanying drawings, in which.

FIG. 1 illustrates schematically a deepwater installation including afloating production and storage vessel and rigid pipeline riser bundlesin a deepwater oil field;

FIG. 2 is a more detailed side elevation of an installation of the typeshown in FIG. 1 including a riser tower according to a first embodimentof the present invention;

FIG. 3 is a cross-sectional view of a riser bundle suitable for use inthe installation of FIGS. 1 and 2;

FIG. 4, 5 and 6 are cross-sectional views of alternative riser bundlearrangements to that shown in FIG. 3;

FIG. 7 is a partial longitudinal cross-section of an insulated flowlinefor use in the riser bundle of FIG. 3 or 4, in which the insulationincludes a pipe-in-pipe structure

FIG. 8 illustrates a modification of the tower of any of the aboveexamples, in which the foam blocks extend only over parts of the tower'slength.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, the person skilled in the art will recognise acut-away view of a seabed installation comprising a number of wellheads, manifolds and other pipeline equipment 100 to 108. These arelocated in an oil field on the seabed 110.

Vertical riser towers constructed according to the present invention areprovided at 112 and 114, for conveying production fluids to the surface,and for conveying lifting gas, injection water and treatment chemicalssuch as methanol from the surface to the seabed. The foot of each riser,112, 114, is connected to a number of well heads/injection sites 100 to108 by horizontal pipelines 116 etc.

Further pipelines 118, 120 may link to other well sites at a remote partof the seabed. At the sea surface 122, the top of each riser tower issupported by a buoy 124, 126. These towers are prefabricated at shorefacilities, towed to their operating location and then installed to theseabed with anchors at the bottom and buoyancy at the top.

A floating production and storage vessel (FPSO) 128 is moored by meansnot shown, or otherwise held in place at the surface. FPSO 128 providesproduction facilities, storage and accommodation for the wells 100 to108. FPSO 128 is connected to the risers by flexible flow lines 132etc., for the transfer of fluids between the FPSO and the seabed, viarisers 112 and 114.

As mentioned above, individual pipelines may be required not only forhydrocarbons produced from the seabed wells, but also for variousauxiliary fluids, which assist in the production and/or maintenance ofthe seabed installation. For the sake of convenience, a number ofpipelines carrying either the same or a number of different types offluid are grouped in “bundles”, and the risers 112, and 114 in thisembodiment comprise bundles of conduits for production fluids, liftinggas, injection water, and treatment chemicals, methanol.

As is well known, efficient thermal insulation is required around thehorizontal and vertical flowlines, to prevent the hot production fluidsoverly cooling, thickening and even solidifying before they arerecovered to the surface.

Now referring to FIG. 2 of the drawings, there is shown in more detail aspecific example of a hybrid riser tower installation as broadlyillustrated in FIG. 1.

The seabed installation includes a well head 201, a production system205 and an injection system 202. The injection system includes aninjection line 203, and a riser injection spool 204. The well head 201includes riser connection means 206 with a riser tower 207, connectedthereto. The riser tower may extend for example 1200 m from the seabedalmost to the sea surface. An FPSO 208 located at the surfaces connectedvia a flexible jumper 209 and a dynamic jumper bundle 210 to the risertower 207, at or near the end of the riser tower remote from the seabed.In addition the FPSO 208 is connected via a dynamic (production andinjection) umbilical 211 to the riser tower 207 at a point towards themid-height of the tower. Static injection and production umbilicals 212connects the riser tower 207 to the injection system 202 and productionsystem 205 at the seabed.

The FPSO 208 is connected by a buoyancy aided export line 213 to adynamic buoy 214. The export line 213 being connected to the FPSO by aflex joint 215.

FIG. 3 shows in cross-section one of the riser towers 112 or 114. Thecentral metallic core pipe is designated C, and is empty, being providedfor structral purposes only. If sealed and filled with air, it alsoprovides buoyancy. Arrayed around the core are production flowlines P,gas lift lines G, water injection lines W and umbilicals U.

Flowlines P and gas lift lines G in this example are coated directlywith an additional insulation material I. This may be a solid coating ofpolypropylene (PP) or the like, or it may be a more highly insulatingmaterial, such as PUR foam or microporous material. PP coating stationsare commonplace, and coatings as tick as 50–120 mm will providesubstantial insulation. The designations C, P, W, G, F, U and I are usedthroughout the description and drawings with the same meaning.

The various lines P, G, W, and U are held in a fixed arrangement aboutthe core. In the illustrated example, the lines are spaced and insulatedfrom one another by shaped blocks F of syntactic foam or the like, whichalso provides buoyancy to the structure.

In general, two cases can be considered:

-   -   Either the insulation requirements (both steady state and cool        down) can be satisfied with the insulation coati, in which case        there is virtually no chance of natural convection developing to        the outside of the line. Expensive gaskets and filler material        are then eliminated    -   Or the insulation must be complemented by another insulating        material such as syntactic foam blocks F.

In the latter case:

-   -   During steady state, the heat transfer loss by natural        convection is nevertheless reduced by the insulation on the        pipes because:    -   The temperature difference is reduced,    -   The effect of heat losses at the junction of two foam blocks is        reduced;    -   At shutdown the thermal inertia of the line, increased by the        thermal inertia of the foam, reduces the heat transfer making it        easier to meet the cooldown time.

In either case, monitoring of the central temperature and pressure canbe easily provided by embedding a Bragg effect optic fibre.

Of course the specific combinations and types of conduit are presentedby way of example only, and the actual provisions will be determined bythe operational requirements of each installation. The skilled leaderwill readily appreciate how the design of the installation at top andbottom of the riser tower can be adapted from the prior art, includingU.S. Pat. No. 6,082,391, mentioned above, and these are not discussed infurther detail herein.

In an alternative embodiment, the core may accommodate some of thelines, and in particular the hot, production flow lines P and/or liftlines G. This is subject of our copending applications GB 0100414.2 andGB 0124802.0 (63753 GB and 63753 GB2). In cases where water convectionin the gaps between the foam blocks F leads to significant heat flow,these gaps can be packed with material such as grease, to preventconvection. This technique is subject of our co-pending applicationnumber PCT/EP01/09575 which claims priority from GB0018999.3 and GB0116307.0, not published at the priority date of the presentapplication.

FIGS. 4 and 5 illustrate two alternative cross-sections where the spaceinside the core is used to accommodate some of the conduits.

In FIG. 4 there is shown a construction of riser having a hollow corepipe C. Located within the core pipe are two production lines P and twogas lift lines G and located outside the core pipe are four waterinjection lines W and three umbilicals U. The spaces between the lineboth internally and externally of the core pipe P are also filled withblocks F of syntactic foam that are shaped to meet the specific designrequirements for the system. It should be noted that in this example thefoam blocks externally located about the core pipe C have been splitdiametrically to fit around the core between the water injection lines,which do not themselves require substantial insulation from theenvironment. There are no insulated lines within the foam outside thecore, and no circumferential gaps between the foam blocks, such as wouldbe required to insulate production and gas lift lines located outsidethe core.

Production flowlines P in this example also carry their own insulationI, being coated with a polypropylene layer, of a type known per se,which also adds to their insulation properties. Relatively thick PPlayers can be formed, for example of 50–120 mm thickness.Higher-insulated foam and other coatings can be used, as explainedbelow.

FIG. 5 of the drawings shows a third example in which only the gas liftlines G are located in the core pipe C, and the production lines P arelocated externally of the core pipe C with the water injection lines Wand umbilicals U. The figure shows the use of foam insulation Finternally of the core pipe C but it will be appreciated that the use ofgrease or wax like material insulation is another options. In thisexample, since the production lines P are closer to the environment andto the water lines, they are provided with enhanced insulation I such asPUR or other foam. Pipe-in-pipe insulation (essentially a double-walledconstruction) is also possible here.

As will be appreciated by those skilled in the art the functionalspecification of the tower will generally require one or two sets oflines, and may typically include within each set of lines twinproduction flowlines to allow pigging and an injection line. A singlewater injection line may be sufficient, or more than one may be provide.

FIG. 6 of the drawings shows in cross-section a simple three-linebundle. In this arrangement the core pipe C supports just two productionlines P and an injection line W which are evenly distributed thereaboutsin a triangular configuration. The lines P. W are surrounded byinsulation blocks F. The need for blocks F to provide insulation isreduced by the coating on the production lines P, reducing the amount offoam material required for insulation purposes. The amount of foam isthereby reduced to what is required for buoyancy and mechanical support.

FIG. 7 of the drawings shows an alternative construction of an insulatedflowline suitable for use with the riser described above as well as inother similar types of applications, this construction for the flowlinecan be described as a “pipe in pipe” arrangement, known per se in theart. This arrangement is generally provided in pre-fabricated sections700 for fitting, for example welding, together and FIG. 7 shows inlongitudinal cross-section the joint between two such sections, whichnaturally extend to left and right of the picture.

Each section comprises a central pipe 701 for the transport of fluidssuch as production fluids and a second pipe 702 in which the pipe 701 ishoused for the major part of its length. Ends 703 of the pipe 701 extendbeyond the second pipe 702 and enable the sections 700 of the pipe 701to be secured together in end to end relationship so as to form apipeline. The second pipe 702 is bent down at its ends 704 to be weldedto the outside of the pipe 701 near to the ends 703 and so defines aspace 705 between the two pipes. This space 705 provides and or housesthe insulation for the pipeline.

In one embodiment a layer 706 of an insulating material, may be providedover the outer surface of the pipe 701 within the space 705. Theinsulating material may be a microporous material; for example ISOFLEX(a Trade Mark of Microtherm) which is a ceramic like material. With thistype of arrangement a gap will still be present between the layer 706and the inner surface of the pipe 702. This space 705 may be a simplespace filled with air or other gas. The pressure in this space 705 maybe normal atmospheric, or a partial vacuum may be created so as toreduce convective heat losses.

In an alternative arrangement the space 705 may be filled with a foammaterial such as a polyurethane foam so as to provide the insulation.

In order to protect and insulate the area around the join in theflowline, it is encased and fixed within a joint 700. The joint 700comprises a sleeve 711 having an outer surrounding sleeve 712 which aswith the section defines a space 714 in which insulating material islocated, for example a layer 714 of ISOFLEX as shown in FIG. 7, orpolyurethane foam, and two heat shrink end collars 710. The sleevearrangement 711, 712 and the heat shrink collars 710 are located aboutone of the sections prior to welding of two sections. When welding iscomplete the component are slid into place about the join in the pipe.An epoxy resin material is injected into the space 707 defined betweenthe sleeve arrangement and the flowline to fill that space. The heatshrink collars 710 are then heated so that they shrink and seal thesleeve arrangement to the flowline.

Any of the insulated flowlines in the embodiments described could be ofpipe-in-pipe construction as just described with reference to FIG. 7 ofthe drawings.

FIG. 8 illustrates a stepped tower construction, compatible with any ofthe examples of FIG. 2, 3 and 4, showing that the foam blocks F need notextend the full length of the tower. In this example the foam insulatingmaterial is provided in discrete sections spaced apart along the lengthof the riser tower. Advantages of the stepped tower include reducedcost, and controllable buoyancy. Another advantage of varying thecross-section along the length of the tower is a reduced tendency tovortex-induced vibration, under the influence of water currents. Inembodiments where some of the warmer lines are outside the core,individual or group insulation of the lines is of course necessary, atleast in the sections between the foam blocks, as in the co-pendingapplication mentioned above.

1. A marine riser tower for use in the production of hydrocarbons fromoffshore wells, wherein a plurality of fluid conduits including at leastone production flow line are supported in a single supporting structure,and at least one of said conduits is provided with its own insulationindependent of said supporting structure; and wherein the insulatedconduit is gas lift line.
 2. A marine riser tower for use in theproduction of hydrocarbons from offshore wells, wherein a plurality offluid conduits including at least one production flow line are supportedin a single supporting structure, and at least one of said conduits isprovided with its own insulation independent of said supportingstructure; and wherein the fluid conduits include at least one waterinjection line.
 3. A marine riser tower for use in the production ofhydrocarbons from offshore wells, wherein a plurality of fluid conduitsincluding at least one production flow line are supported in a singlesupporting structure, and at least one of said conduits is provided withits own insulation independent of said supporting structure; wherein theriser tower has a tubular core, and said core accommodates some of theconduits and not others; and wherein the core accommodates a pluralityof gas lift lines, while associated production lines are individuallyinsulated and located outside the core.
 4. A marine riser tower for usein the production of hydrocarbons from offshore wells, wherein aplurality of fluid conduits including at least one production flow lineare supported in a single supporting structure, and at least one of saidconduits is provided with its own insulation independent of saidsupporting structure; and wherein said insulation includes a coatingapplied to the at least one conduit.
 5. A marine riser tower for use inthe production of hydrocarbons from offshore wells, wherein a pluralityof fluid conduits including at least one production flow line aresupported in a single supporting structure, and at least one of saidconduits is provided with its own insulation independent of saidsupporting structure; buoyant material surrounding the riser as a wholeat least at some points along its length; and wherein said buoyantmaterial is provided as foam blocks spaced along the length of theriser.
 6. A marine riser tower for use in the production of hydrocarbonsfrom offshore wells, wherein a plurality of fluid conduits including atleast one production flow line are supported in a single supportingstructure, and at least one of said conduits is provided with its owninsulation independent of said supporting structure; buoyant materialsurrounding the riser as a whole at least at some points along itslength; and wherein foam material is provided in discrete sectionsspaced apart along the length of the riser.
 7. A marine riser tower foruse in the production of hydrocarbons from offshore wells, wherein aplurality of fluid conduits including at least one production flow lineare supported in a single supporting structure, and at least one of saidconduits is provided with its own insulation independent of saidsupporting structure; and wherein flexible lines are connected to theriser at top and/or bottom.